As wireless communications become more widely used, the demand for limited wireless resources, such as the finite number of frequency bands, time divisions, and/or identifying codes (collectively referred to herein as xe2x80x9cchannelsxe2x80x9d) have increased significantly. It should be appreciated that channels may be distinguishable based on the particular air interface standard implemented such as the frequency bands of frequency division multiple access (FDMA), time slots of time division multiple access (TDMA), codes (pseudo random, Walsh, Orthogonal Variable Spreading Factor, etc.) of code division multiple access (CDMA), and the like. In order to more efficiently use these available resources, wireless communication systems typically divide a geographic area into multiple overlapping coverage cells, which are each served by a base station. Each base station typically comprises a tower, one or more antenna, and radio equipment to allow wireless communication devices to connect with the network side of a wireless communications link.
The planning process which defines the deployment and growth of mobile radio networks with respect to forecasted demand usually precedes their operation and management. The planning department uses predictions of traffic and propagation environment to determine the adequate placement of base stations (BTSs), each having a plurality of transceivers, in the intended service area, as well as their configuration. This configuration encompasses issues like power class, antenna type, antenna pointing, or frequency plan, and it results in a large number of parameters that need to be set. Some of these parameters cannot be easily changed once a decision is made (for instance, changing a base station location once the tower is built), whereas other parameters allow changes through simple software updates (for instance, changing the carrier frequency).
Once the planning department has decided on a configuration for the service area, the operations department deploys the plan and the system can go live. At this stage, actual performance measurements can be collected (either through drive-tests, handset measurements, or switch statistics) and fed back to the planning department to validate the predictions. If discrepancies are found (usually in the form of impaired service quality), the planned configuration is fine-tuned and a new configuration is returned to the operations department for deployment. The fine-tuning process is iterated periodically to improve system performance and also to track any changes (for instance, an unexpected increase in volume of calls) that would require a major configuration update.
In any radio mobile networks that rely on frequency reuse, one of the key parameters that need to be optimized is the set of carrier frequencies allocated to each BTS. The reason for the need to allocate frequencies in these networks is that frequencies cannot be universally reused at each BTS without incurring unacceptable interference levels. The license granted a cellular system operator is limited to a finite number of carrier frequencies for use by that operator. Therefore a decision has to be made as to which frequencies can be used in which BTSs so that the interference levels provide acceptable quality, while at the same time maximizing capacity per carrier frequency (by reusing the frequencies as tightly as possible).
Assuming that the number of frequencies for each BTS in a set has already been defined through traffic engineering dimensioning, the frequency allocation problem (FAP) can be defined as the allocation of a finite set of frequencies to BTSs such that 1) each BTS gets the required number of frequencies, 2) a set of frequency separation constraints is satisfied, and 3) a certain cost function (usually an aggregate interference measure) is minimized. The frequency separation constraints preclude frequency combinations that are known to generate excessive interference (like using adjacent frequencies in the same BTS), or that cannot be used due to regulations or agreements with other operators (like in country border areas). A first approximation to an automatic frequency allocation system can be found in U.S. Pat. No. 5,212,831 by Chuang and Sollenberger, entitled xe2x80x9cMethod and apparatus for autonomous adaptive frequency assignment in TDMA portable radio systemsxe2x80x9d, where BTSs choose the frequency to use based on downlink measurements performed at the very same BTS. The drawbacks of this approach are that the BTS has to stop operation to be able to perform the measurements, the antennas have to be omnidirectional to assure algorithm convergence and to avoid temporary service disruption in other base stations, and the configuration converges to a local optimum (that is, a solution that is the optimum in a certain neighborhood of feasible solutions, but that is not optimum over the whole set of feasible solutions).
Another solution proposed in the prior art is described in U.S. Pat. No. 5,966,657 by Sporre, entitled xe2x80x9cMethod and System for Radio Frequency Measurements and Automatic Frequency Planning in Cellular Radio System.xe2x80x9d In this solution, the BTS sends a message to the mobile stations within its cellular area telling them on which frequencies to measure the signal strength. The mobile station then measures the signal strength for each of the selected frequencies. The mobile stations then report back to their base stations the measured signal strength levels. The measured signal strengths for each frequency measured at the mobile station is combined with the signal strength for measure for each frequency at the base station. A composite signal strength for each frequency is calculated. The composite signal strengths for each carrier frequency are then used by the cellular system to determine whether any of the non-allocated frequencies are less interfered with than the allocated frequencies. The system then uses this information to update the system frequency plan to improve the quality of service within any cell. However, this system suffers from the fact that the process only uses measured data from a single cell area and therefore produces an optimized frequency selection for a single cell; in other words a frequency allocation that is locally optimized. In addition, because the composite signal strength for any one frequency combines the signal strength measured both at the mobile station and the base station, the composite signal does not, in application, provide a valid measure of interference in systems with sectorized antennas. It only provides an accurate local optimum in systems having omnidirectional BTSBTS antennae.
Therefore it is an object of the present invention to obtain a frequency allocation for a plurality of cells that provide a system-wide optimum that overcomes the inadequacies and deficiencies of the prior art.
Our invention is a method and system for collecting measurements from different parts of a cellular network, generating statistics from these measurements, and combining these statistics with inventory, propagation and geographic data. The combined data is used as input to a frequency allocation algorithm. As a result, the allocation algorithm produces a frequency configuration set, a quality measure of the configuration proposed and/or a set of recommendations that is fed back to the cellular base stations to re-allocate the frequency usage in a plurality of cells.